1. Field of the Invention
The present invention relates to a field-emission type electronic device containing an electron source which is operated to emit electrons on the principle of field emission, and more particularly to a cold cathode provided in the field-emission type electronic source.
2. Description of the Related Art
In recent days, a remarkable progress has been made about a technique for manufacturing the field-emission type electronic device for emitting electrons in a high electric field in vacuum as a result of developing a fining technique utilized in the field of an integrated circuit or thin film deposition. In particular, a field-emission type cold cathode having a quite fine structure has been manufactured. This type of field-emission type cold cathode is the most fundamental electron-emission device included in the essential parts of a micro electronic tube or electron gun.
The field-emission type electronic device or the field-emission type electron source containing a lot of electron-emission devices has been invented for an essential component for a micro triode or a thin display element, for example. The operation and the manufacturing method of the field-emission type electronic device or the field-emission type electron source have been known in the technical report: C. A. Spindt, et. of Stanford Research Institute, pp. 5248 to 5263, Vol. 47, December (1976) of Journal of Applied Physics. Further, they have been disclosed in U.S. Pat. Nos. 4,307,507 and 4,513,308 invented by H. F. Gray, et.
Then, some of the related arts will be described later.
In a conventional field-emission electron source, a substrate electrode is formed of monocrystalline silicon having low resistance in order to keep compatibility with a fining technique in the field of an integrated circuit or thin film deposition, lower the cost, and make it monolithic. On the substrate electrode, a lot of conical cold cathode chip are formed. Each cold cathode chip is made of the same monocrystalline silicon as the substrate electrode or a high melting point metal such as tungsten (W) or molybdenum (Mo). An insulating layer is formed on the substrate electrode around the cold cathode chip. On the insulating layer, a gate electrode is deposited. An anode electrode is provided to cover those cold cathode chips and the gate electrode as keeping vacuum space between the anode electrode and the side of the cold cathode chips and the gate electrode.
In such a electron source, a voltage of about 100 to 200 V is applied as a gate voltage between each cold cathode chip and the gate electrode. The application results in causing a strong electric field of about 10.sup.7 V/cm between each cold cathode chip and the gate electrode, thereby allowing each cold cathode chip to emit electrons on the field-emission principle. The anode voltage of 300 to 500 V applied to the anode electrode causes emitted electrons to reach the anode electrode.
In the current techniques, the critical diameter of the conical cold cathode chip is about 1 .mu.m and the critical height thereof is about 1 .mu.m. Further, it is practically impossible to avoid variable electron-emission characteristics in those chips caused by the variations of the cold cathode chips. To overcome the disadvantageous matter, the anode electrode is made of a transparent material and a fluorescent material is coated on the transparent anode electrode. A trial is now being made for a thin display unit using the cold cathode chips as electron-emission sources only. In a case that this type of field-emission electronic device applies to the thin display unit, it is unnecessary to accurately control the emitted electrons. Hence, 1000 or more electron-emission cold cathode chips, which are arranged per one pixel in an array manner, are driven in parallel for the purpose of averaging the variation of the electron-emission cold cathode chips and obtaining the necessary amount of emitted electrons.
In a case that the field-emission cold cathode chips are used for a micro triode, the resulting triode may break the shortcomings and the limits entailed in the solid device such as a semiconductor device. The solid device has such a limit that the saturated traveling speed of electrons in the solid device is about c/1000 (c is a light speed). On the other hand, in the field-emission electronic device, the emitted electrons travel in vacuum. Hence, the traveling speed of the electrons may be faster than the traveling speed of the electrons in the solid device by one or more digits. Further, the field-emission electronic device is more endurable in high temperature and radioactive rays. For example, in a case that a voltage of 50 V is applied between the electrodes keeping a spacing of 1 .mu.m therebetween, the traveling speed of electrons is 2.times.10.sup.8 cm/s on average and the traveling time for a distance of 1 .mu.m is 0.5 psec.
The use of the triode having dimensions on sub-micron order, therefore, makes it possible to realize a super high-speed device having a response speed on tera-hertz level.
In the known field-emission type electron source, a field-emission type cold cathode chip is formed like a conical form on a substrate electrode made of a metal or semiconductor material as mentioned above. An insulating layer is formed to cover the substrate electrode around the field-emission type cold cathode. On the insulating layer, a gate electrode is deposited. When a voltage is applied between the field-emission type cold cathode and the gate electrode, a high electric field takes place between the cold cathode and the gate electrode so that electrons can be emitted from the field-emission cold cathode on the basis of the field-emission principle.
The field-emission cold cathode is made of silicon or metal such as tungsten (W) or molybdenum (Mo). Further trial is now being made for optimizing the form of the field-emission cold cathode in order to reduce an operating voltage on which electrons are emitted.
In another conventional field-emission electron source, like the foregoing composition, a field-emission cold cathode is formed like a conical form on a substrate electrode. An insulating layer is formed on the substrate electrode around the field-emission cold cathode. On the insulating layer, a gate electrode is deposited. The substrate electrode is made of semiconductor or metal. Unlike the foregoing composition, the substrate electrode is projected like a pyramid at the site where the conical field-emission cold cathode is to be formed. On the pyramid portion, a coating layer is deposited. The coating layer is made of a material having a low work function such as cesium (Cs) or lanthanum hexabolaide (LAB6). It means that the pyramid portion of the substrate electrode and the coating layer deposited thereon compose the field-emission cold cathode.
Next, the shortcomings of the conventional compositions will be described.
For the known field-emission electric devices, the following shortcomings take place. Since the distance between the cold cathode chip served as a cathode electrode and the gate electrode is not made so small, it is necessary to apply a large voltage between the cathode electrode and the gate electrode for obtaining the necessary electric field to allowing the tip of the cold cathode chip to emit electrons. Further, since the distance between the cathode electrode and the anode electrode is made larger, it needs a considerable time to travel electrons between the cathode electrode and the anode electrode.
The cold cathode chip has a cut-off frequency f.sub.T represented by the express ion: EQU f.sub.T =g.sub.m /(2.pi.C.sub.gc)
wherein g.sub.m is a mutual conductance and C.sub.gc is a capacitance between the gate electrode and the cathode electrode.
To realize a cold cathode chip enabling to operate at high speed, therefore, it is necessary to increase the mutual conductance g.sub.m but decrease the capacitance C.sub.gc. However, in the structure of the known field-emission electronic devices, the electron emission is made possible only at the tip of the cold cathode chip. Further, since it is difficult to make the spacing between the adjacent cold cathode chips small in light of the manufacturing technique, the area where electrons are emitted and the amount of emitted electrons are both small. Hence, it is difficult to increase the mutual conductance g.sub.m of the electronic device depending on the current density of the field emission. Further, the field-emission electronic devices has the structure where the gate electrode layer is opposed to the cathode electrode layer as keeping the insulating layer therebetween. The structure inevitably increases the value of the capacitance C.sub.gc between the gate electrode and the cathode electrode.
In turn, for the first conventional field-emission electron source, in a case that the field-emission cold cathode is made of a high melting point metal such as tungsten (W), molybdenum (Mo) or titanium (Ti), those metals are thermally endurable and mechanically strong, but have so high work functions. For example, the work function of tungsten is about 4.3 eV and one of molybdenum is about 4.2 eV. They disadvantageously need high operating voltages.
For the second known composition of a field-emission electron source as mentioned above, the work function of the coating layer is so low such as about 2.1 eV in case of using cesium (Cs) and about 2.7 eV in case of using lanthanum hexabolaid (LaB.sub.6). Hence, the operating voltage is made smaller. The difference of thermal expansion coefficient between the material of the coating layer and the material of the substrate electrode causes the resulting cold cathode to be thermally unstable and mechanically weak. Since the material of the coating layer is chemically active, a shortcoming takes place that the work function is subject to change. Additional, since the material of the coating layer such as selenium has a far larger than the substrate electrode made of metal or semiconductor, the electric conduction between both is made worse, so that the electron emission is difficult to take place.